A Guide To Endoscopic Decompression For Intermetatarsal Neuromas

When conservative measures fail, endoscopic decompression may provide pain relief of Morton’s neuroma. This author provides key diagnostic pearls, imaging insights and surgical tips.

There are many causes of intermetatarsal neuroma pain or generalized pain in the plantar forefoot area. The common source of such relentless pain is a peripheral nerve affliction, which may affect one or more digital plantar nerves as they course through the intermetatarsal spaces.

   Morton’s neuroma is a benign lesion or enlargement of the third common digital branch of the medial plantar nerve. The neuroma is located between and is often located distal to the third and fourth metatarsal heads.

   Classically, the nerve passes underneath the deep transverse intermetatarsal ligament on its plantar aspect. There are other structures that also pass through along with the nerve. These structures are the third plantar metatarsal artery, vein and the tendon slip from the third lumbrical muscle. This muscle inserts into the extensor hood apparatus on the medial aspect of the fourth toe.

   In addition to the third intermetatarsal space, an intermetatarsal neuroma may also occur in other locations such as the second intermetatarsal space but rarely within the fourth or the first. It is important to note that intermetatarsal neuromas do occur bilaterally and one must treat them accordingly.

Key Diagnostic Pointers

The most common location for Morton’s neuroma is the distal third intermetatarsal space. It occurs more frequently in women than in men and affects those ranging in age from 18 to 60. The patient is likely to be overweight.1 In moderate cases, the patient describes the pain as a sensation of walking on a wrinkle or a lump in the sock or shoe.

   During the physical examination, you may note some swelling of the plantar metatarsal surface or sulcus in comparison with the contralateral foot. It is clinically prudent to pay attention to the splaying of adjacent toes. If the neuroma is large enough, the adjacent toes may be forced to spread apart on weightbearing.

   One can best diagnose Morton’s neuroma by simply listening to the patient’s symptoms. Patients have described the pain from Morton’s neuroma as sharp, dull or throbbing. However, it classically manifests as a paroxysmal burning sensation similar to “walking on a hot pebble.” The pain is most often localized in the third and fourth plantar metatarsal heads, but may radiate to distally into adjacent toes, especially the fourth, transversely to adjacent metatarsal heads. Pain may also manifest proximally up the leg to the knee and, in rare instances, as high as the hip.

   Walking in shoes significantly exacerbates the pain and patients report relief to a degree after removing the shoes. A characteristic sign of painful Morton’s neuroma is one’s desire to remove the shoes, rub the forefoot and flex the toes. This usually provides immediate relief though it is transient.

   One can reproduce pain by squeezing each intermetatarsal space in a dorsoplantar direction at or distal to the metatarsophalangeal joints. One must differentiate local tenderness from arthritic type pain and/or stress fractures. Mulder’s sign is positive when lateral compression of the forefoot combined with plantar and dorsal pressure produces a silent, palpable and sometimes painful click in the affected intermetatarsal space.2

   The differential diagnosis of Morton’s neuroma includes:
* metatarsal stress fracture
* rheumatoid arthritis
* osteochondritis dissecans or Freiberg’s infraction
* localized vasculitis
* ischemia
* tarsal tunnel syndrome
* nerve root compression syndromes
* peripheral neuritis
* peripheral neuropathy
* intermetatarsal bursitis

   Radiographs. Obtain weightbearing radiographs in order to rule out bony abnormality.

   Sensory nerve conduction study. Electrodiagnostic techniques are not very accurate. This is primarily due to difficulty in isolating a single interdigital nerve with an electrode to measure sensory conduction velocity.

   Magnetic resonance imaging (MRI). Magnetic resonance imaging is very useful in differentiating Morton’s neuroma mass from adjacent surrounding bone and soft tissues.

Pertinent Insights On The Etiology Of Morton’s Neuroma And Biomechanics

Researchers have proposed many theories for the etiology of Morton’s neuroma.1,3 Histological findings leave little doubt that the syndrome is indeed a mechanical entrapment neuropathy. The deep transverse intermetatarsal ligament appears to be the anatomic structure against which most of this entrapment occurs. The majority of intermetarasal neuromas occur in the pronated foot, where there are not only excessive stretch forces imposed on the interdigital nerves but also compressive and shearing forces form the adjacent hypermobile metatarsal heads.

   As the medial and lateral plantar nerves pass along the medial side of the foot, they dive plantarly under the arch. Stress occurs on these nerves during prolonged midstance pronation as the foot everts, abducts and dorsiflexes. Tension increases as these nerves travel around the flexor digitorum brevis sling and draw up tightly against the plantar and anterior edge of the unyielding deep transverse intermetatarsal ligament. Further tension and compression occur at this ligament when the toes hyperextend at the metatarsophalangeal joint. Thus, occupations requiring repetitive toe hyperextension can result in the development of an intermetatarsal neuroma, regardless of foot type.

   Finally, pointed toe or narrow shoes can further add pressive forces that favor the production of intermetatarsal neuromas, regardless of foot type. High-heeled shoes throw weight forward onto the ball of the foot, jamming it into the narrow front of the shoe. These shoes will also force the toes into hyperextension and thus contribute to the etiology.

A Guide To Surgical Management

When physicians have exhausted conservative treatment without relief of pain, surgical intervention is indicated.

   One option for neuroma management is endoscopic decompression of intermetatarsal neuroma. The surgeon would place the patient on the operating table in the supine position. After ensuring appropriate anesthesia, one would prep and drape the affected foot in the appropriate fashion. Apply a thigh or ankle tourniquet. Inflate it to 250 to 300 mmHg of pressure after appropriate elevation and exsanguination utilizing an Esmarch bandage.

   Mark the affected interspace to ensure proper cannular placement. Place a longitudinal line over the dorsal aspect of the interspace between the respective metatarsal heads from proximal to distal. Palpate the metatarsal heads and place a dorsal incision approximately 2.5 cm proximal to the level of the metatarsophalangeal joint.

   Utilizing a #15 blade, make a small 4 mm transverse incision on the dorsal aspect of the interspace delineated by the markings. Pay careful attention not to transect tissue deeper than the dermis. One may achieve protection of the superficial peroneal nerve branches by utilizing blunt dissection with Stevens tenotomy scissors to make the portal deeper between the respective metatarsal heads. Place a metatarsal retractor into the portal and gradually retract the instrument. This causes tension on the transverse intermetatarsal ligament.

   Make another incision in the affected webspace in a transverse manner between the dorsal and plantar neurovascular bundles. Deepen the incision with blunt dissection using tenotomy scissors down to the level of the intermetatarsal ligament. Use the elevators to create a channel underneath the transverse intermetatarsal ligament for placement of the cannula. Note that one identifies proper tissue planes by the minimal resistance on the instrument during its placement. After placing the elevator inferior to the transverse intermetatarsal ligament, place dorsal pressure on the instrument to feel the unforgiving rigidity of the transverse intermetatarsal ligament.

   At this time, place the oval cannula in the same position beneath the transverse intermetatarsal ligament that the elevator achieved. Remove the obturator and leave the cannula in place. Placement of a 2.7 mm, 30 degree beveled scope allows for visualization on the monitor of the transverse intermetatarsal ligament. One can identify the transverse intermetatarsal ligament’s proximal border and differentiate the ligament from the tissue proximal to it.

   At this point, introducing an angled hook blade in the same cannula, one transects the transverse intermetatarsal ligament from proximal to distal into two nice, separate edges. Then endoscopically identify the ligamentous edges, which separate upon placement of further retraction on the intermetatarsal retractor. At this point, one must take photographs. Then place the obturator in the cannula and remove the obturator-cannula instrumentation from the interspace. Reintroduce the elevator into the portal incision. Upon inspection, there should be no evidence of any resistance against the elevator due to the transected intermetatarsal ligament.

   Then perform closure of the two incisions. One may use a corticosteroid along with local anesthetic. Dressing occurs in the usual manner. Patients may ambulate immediately after the procedure in a postoperative shoe.

In Conclusion

One study has shown that satisfactory results with the endoscopic decompression of intermetatarsal neuroma occur in 88 percent of patients versus 74 percent of patients who underwent neurectomy.4 There have been no serious complications reported as a result of this procedure. Although the learning curve is slightly high, the results produced by endoscopic decompression are far superior to open neurectomy with far fewer complications such as “stump neuroma.”

   It is prudent to note that the proper patient selection is key to the success of this minimally invasive procedure. One must always exercise conservative treatment prior to any surgical intervention. However, when non-surgical means fail to alleviate the patient’s complaints and symptoms, endoscopic decompression is advisable.

   Dr. Fanous is the President as well as a Fellow and the Co-Scientific Chairman of the Academy of Ambulatory Foot and Ankle Surgery. He is the President and CEO of Advanced Medical Management, a medical consulting firm. Dr. Fanous is a former founder and Director of Podiatric Medical Education and Residency Training Program at Anaheim General Hospital in Anaheim, Calif. He is in private practice in Southern California.

   Dr. Fanous notes the opinions and assertions contained herein are his views and should not be construed as reflecting the views of the Academy of Ambulatory Foot and Ankle Surgery.

References

1. Bartolomei FJ, Wertheimer SJ. Intermetatarsal neuroma: distribution and etiologic factors. J Foot Surg. 1983; 22(4):279-282.
2. Mulder JD. The causative mechanism in Morton’s metatarsalgia. J Bone Joint Surg. 1951; 33B(1):94-95.
3. Tate RO, Rusin JJ. Morton’s Neuroma: its ultrastructure anatomy and biomechanical etiology. J Am Podiatry Assoc. 1978; 68(12):797-807.
4. Addante JB, Peicott PS, Wong KY, Brooks D. Interdigital neuromas. Results of surgical excision of 152 neuromas. J Am Podiatr Med Assoc. 1986; 76(9):493-495.
Additional References
5. Silverman LJ. Morton’s toe or Morton’s neuralgia. J Am Podiatry Assoc. 1976; 66(10):749-756.
6. Alexander IJ, Johnson KA, Parr JW. Morton’s neuroma: a review of recent concepts. Orthopedics. 1987; 10(1):103-106.
7. Miller SJ. Morton’s neuroma, A syndrome. In: McGlamry ED, Banks AS, Downey MS, eds. Comprehensive Textbook of Foot Surgery. Williams and Wilkins, Baltimore, Chapter 11, pp. 304-317, 1987.
8. Berlin SJ, Donick II, Block LD, Costa AJ. Nerve tumors of the foot: diagnosis and treatment. J Am Podiatry Assoc. 1975; 65(2):157-166.
9. Gauthier G. Thomas Morton’s disease: a nerve entrapment syndrome- a new surgical technique. Clin Orthop. 1979; 142:90-107.
10. Bossley CJ, Cairney PC. The intermetatarso-phalangeal bursa-its significance in Morton’s metatarsalgia. J Bone Joint Surg. 1980; 62B(2):184-191.